The invention described herein relates to a process for the production of (R) and (S)-aminocarnitine and its derivatives starting from D- and L-aspartic acid.
Aminocarnitine is a substance endowed with interesting pharmaceutical properties and its N-derivatives arouse a similar degree of interest. For example, D. L. Jenkins and W. O. Griffith have described the antiketogenic and hypoglycaemic effects of the N-acetylates in the racemic form. U.S. Pat. No. 4,521,432 (Takeda) describes the possible applications of (xe2x88x92)-N-acetyl-aminocarnitine, inner salt, in the treatment of the complications of diabetes. Similar activity has been described for (+)-aminocarnitine, chloride hydrochloride. It would therefore be of interest to have processes for the preparations of the enantiomorph, which match up to the criteria of economic convenience on an industrial scale.
R(+)-aminocarnitine is obtained via hydrolysis of R-(xe2x88x92)-N-acetylcarnitine, the latter being isolated by the cultivation of micro-organisms of the genera Emericella or Aspergillus, or, alternatively, via a complex chemical process described in the Takeda patent cited above.
Other methods of chemical synthesis are known, all rather complex, such as, for example, the one described by Shinagawa, J. Med. Chem., 30; 1458 (1987), who uses diazomethane, which is known to be hazardous. In any event, this method is not of industrial interest, in that it was conceived in order to ascertain the absolute configuration of the single enantiomorph.
The single enantiomorphs can also be obtained by resolution of the racemic mixture of (xc2x1)-N-acetylaminocarnitine, as described in EP 0 287 523.
Alternatively, R(+)- and S(xe2x88x92)-aminocarnitine chloride can be obtained by resolution on silica gel chromatography or fractional crystallisation of the respective N-xcex1-methylbenzyl, benzylester chlorides, as described in Italian patent 1,231,751. This process, which involves subsequent debenzylation is laborious and not very suitable for industrial-scale production.
A method is also known using chiral carnitine as a starting product (Journal of Organic Chemistry, 1995, 60, 8318-8319; (Sigma-Tau) EP 636603, 1995). This method uses reagents such as methane-sulphonic anhydride and sodium azide and solvents such as anhydrous dimethylsulphoxide, and involves a catalytic reduction step.
A process has now been found for the preparation of single enantiomorphs starting from D-aspartic acid and L-aspartic acid, respectively, with an overall yield of at least 38% in 6 to 7 steps, but without it being necessary to purify the intermediates. In pratice, the process according to the invention described herein is realised via direct hydrolysis of the chiral aminocarnitine ester in an acidic milieu to yield a chiral aminocarnitine inner salt without purifying the intermediate products. The enantiomeric purity of the aminocarnitine thus obtained is xe2x89xa799%.
Thus, an object of the invention described herein is a process for the preparation of (R) and (S)-aminocarnitine and of a number of its N-substituted derivatives. In particular, the invention described herein provides a process which also enables aminocarnitine derivatives to be obtained which are useful for the preparation of medicaments for the treatment of diseases associated with hyperactivity of carnitine palmitoyltransferase.
These derivatives are described in Italian patent application MI98A001075, filed on May 15, 1998, and in international patent application PCT/IT99/00126, filed on May 11, 1999, both of which in the name of the applicant and incorporated herein for reference purposes.
The process according to the invention described herein allows the preparation of compounds with the following formula: 
in which
Y is hydrogen or one of the following groups:
xe2x80x94R1,
xe2x80x94COR1,
xe2x80x94CSR1,
xe2x80x94COOR1,
xe2x80x94CSOR1,
xe2x80x94CONHR1,
xe2x80x94CSNHR1,
xe2x80x94SOR1,
xe2x80x94SO2R1,
xe2x80x94SONHR1,
xe2x80x94SO2NHR1,
xe2x80x83where
R1 is a straight or branched, saturated or unsaturated alkyl containing from 1 to 20 carbon atoms, optionally substituted with an A1 group, where A1 is selected from the group consisting of halogen, C6-C14 aryl or heteroaryl, aryloxy or heteroaryloxy, which can optionally be substituted with straight or branched, saturated or unsaturated lower alkyl or alkoxy, containing from 1 to 20 carbon atoms, halogens;
said process comprises the following steps:
a) conversion of D-aspartic or L-aspartic acid to Nxe2x80x94Y substituted D-aspartic or L-aspartic acid;
b) conversion of the Nxe2x80x94Y substituted D-aspartic or L-aspartic acid to the respective anhydride;
c) reduction of the anhydride obtained in step b) to the corresponding 3-(NHxe2x80x94Y)-lactone;
d) opening of the lactone obtained in step c) to yield the corresponding D- or L-3-(NHxe2x80x94Y)-amino-4-hydroxybutyric acid;
e) transformation of the 4-hydroxy group of the D- or L-3-(NHxe2x80x94Y)-amino-4-hydroxybutyric acid into a leaving group;
f) substitution of the end group in position 4 of the D- or L-3-(NHxe2x80x94Y)-aminobutyric acid with a trimethylammonium group;
g) hydrolysis of the ester group; and, if so desired,
h) restoration of the amino group.
The usefulness of this new synthesis route, as compared to the method involving the use of chiral carnitine as the starting product (Journal of Organic Chemistry, 1995, 60, 8318-8319; EP 0 636 603 (Sigma-Tau)), consists in the fact that the use of reactants such as methane-sulphonic anhydride and sodium azide, of dimethyl-sulphoxide as a solvent, and of a catalytic reduction step is avoided. What is more, the volumes involved are lower, thus allowing better management of the reactions and of any purification of intermediate products. In fact, the process according to the invention presents the additional advantage that all steps can be carried out avoiding purification of the intermediates, without this jeopardising the purity of the end product. This advantageous characteristic is obvious to the expert in the art; in particular, the fact will be appreciated that no purification operations are necessary which would place an additional burden on the synthesis process in terms of economic costs, time, materials, specialised personnel and equipment.
As compared to the process described in Journal of Medicinal Chemistry, 1987, 30, 1458-1463 (Takeda), involving the use of benzyloxycarbonyl-L-asparagine as the starting product (with 7 steps and a 24% overall yield), the advantage at industrial level of avoiding reactants such as diazomethane, silver benzoate and dimethyl-sulphate appears obvious. In another process (Bioorganic and Medicinal Chemistry Letters, 1992, 2 (9), 1029-1032), (R)-aminocarnitine is obtained starting from a derivative of aspartic acid (the tert-butylester of N-benzyloxycarbonyl-L-aspartic acid) in seven steps with a yield of 24%-22%, but again using reactants such as diazomethane and silver benzoate, a catalytic hydrogenation step, and methylation with methyl iodide.
The process which is the subject of the invention described herein is described in the following reaction scheme, which exemplifies the case of R(+)-aminocarnitine. It is absolutely obvious to the expert in the sector that the case of S-(xe2x88x92)-aminocarnitine is equally described by the scheme and that no modification is necessary, apart from the fact that the starting compound is of the opposite configuration, namely S-(xe2x88x92)-aspartic acid. 
In the context of the invention described herein, examples of the straight or branched C1-C20 alkyl group are methyl, ethyl, propyl, butyl, pentyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl and eicosyl and their possible isomers, such as, for example, isopropyl, isobutyl and tert-butyl.
Examples of the (C6-C14) aryl, or (C6-C14) aryloxy, heteroaryl or heteroaryloxy group, possibly substituted with straight of branched alkyl or alkoxy with from 1 to 20 carbon atoms, said alkyl group being as exemplified above, are phenyl, 1- or 2-naphthyl, anthracenyl, benzyl, 2-phenylethyl 1-phenylethyl, 3-phenylpropyl, 2-anthracenylpropyl, 1-anthracenylpropyl, naphthylmethyl, 2-naphthylethyl, 1-naphthylethyl, 3-naphthylpropyl, 2-naphthylpropyl, 1-naphthylpropyl, cyclohexylmethyl, 5-phenylpentyl, 3-phenylpentyl, 2-phenyl-3-methylbutyl, thienyl, quinolyl, pyridyl, 5-tetrazolyl, and the equivalent ether derivatives.
What is meant by halogen is fluorine, chlorine, bromine, or iodine.
In a first embodiment of the invention described herein, the process involves steps a)-g), and optionally h), described above. According to this first realisation, and with reference to the scheme given above, commercial chiral aspartic acid 1 is treated with a reactant suitable for introducing the Y group on the nitrogen atom. This step both functions to protect the amino group in the subsequent steps of the process and, if suitably selected, represents the group which will be present in the end compound, according to the meanings attributed above to the Y group.
Assuming that, in the end compound, the Y group is other than hydrogen, different cases may be envisaged in the process according to the invention.
In the case in which Y is R1, the substitution reaction of a hydrogen of the amino group takes place by reaction with alkancarbaldehydes, where the alkyl portion is a homologue of the R1 group desired, and subsequent reduction.
When Y is xe2x80x94COR1, xe2x80x94CSR1, xe2x80x94COOR1, xe2x80x94CSOR1, xe2x80x94CONHR1, xe2x80x94CSNHR1, xe2x80x94SOR1, xe2x80x94SO2R1, xe2x80x94SONIIR1 and xe2x80x94SO2NHR1, the compounds are obtained by reactioin with acyl chlorides, thoacyl chlorides, alkyl chloroformates, alkyl thiochloroformates, alkyl isocyanates, alkyl thioisocyanates, alkyl sulphinyl chlorides, alkyl sulphonyl chlorides, SOCl2 and alkyl amines, alkyl sulphamoyl chlorides (or SO2Cl2 and alkyl amines), containing the desired alkyl R1 group.
As regards the different meanings of R1, present in the various reactants, these are available commercially, or can be prepared according to known methods described in the literature, to which the expert in the art may refer, completing his general knowledge of the subject.
In a second embodiment of the invention described herein, the process involves steps a)-c), and then a step cxe2x80x2), that is to say the opening of the lactone with the introduction of a leaving group X, followed by step g), and optionally h), described above.
In a third embodiment of the invention described herein, the process requires that step f), which has been reached according to one of the first two embodiments of the invention, be followed by step i), i.e. the direct transformation of the ester of the Nxe2x80x94Y substituted aminocarnitine to aminocarnitine.
In a preferred form, and by way of an example, commercial chiral aspartic acid 1 is protected to yield derivative 2. Protective groups (Y in the scheme) are well known and require no particular description. As an example, we may cite the tosyl group, which, in the reaction envisaged in the invention, is described in Helv. Chim. Acta 1996, 79, 1203-1216, or the benzyloxycarbonyl group, which, in the reaction envisaged in the invention, is described in J. Am. Chem. Soc. 1986, 108, 4943-4952. Thus, derivative 2 is cyclised to anhydride 3, as described, for example, in Helv. Chim. Acta 1994, 77, 2142-2146, and subsequently reduced to lactone 4 (see Helv. Chim. Acta 1994, 77, 2142-2146).
Compound 4 can be transformed into compound 5a by treatment with an alcohol ROH, where R is a straight or branched 1 to 14 carbon alkyl, or an arylalkyl, e.g. methanol, isobutanol or benzyl alcohol, in the presence of a suitable transesterification catalyst, such as, for example, an acid or a base (also in the form of resin), preferably amine, such as trimethylamine. By treatment with a reactant suitable for transforming the hydroxyl into an end group, e.g. alkyl or arylsulphonyl chlorides, such as methane sulphonyl chloride in pyridine, triflic anhydride, 5a yields 5b, which by reaction with trimethylamine yields 6. Aminocarnitine can be obtained from 6 by hydrolysing the ester and deprotecting the amino group according to customary procedures.
In accordance with the second embodiment of the process according to the invention, step cxe2x80x2) involves the opening of the lactone with iodotrimethylsilane, described in the literature when ethanol is used as the alcohol (Helv. Chim. Acta, 79, 1996, 1203-1216) and makes it possible to obtain the iododerivative 5b (X=iodine) with good yields. Similar lactone opening reactions can, of course, be easily envisaged with other leaving groups.
Thus, intermediate 5b is treated in a nucleophilic substitution reaction with trimethylamine to yield intermediate 6, which, by alkaline hydrolysis and subsequent deprotection of the amine group supplies the desired product, e.g. on deprotecting with 48% HBr, dibromohydrate is obtained. After a step on IRA 402 resin (OHxe2x88x92) aminocarnitine inner salt 8 is obtained.
According to the third embodiment of the invention described herein, on proceeding directly to the acid hydrolysis of 6 to give 8, the overall yield rises to 38% in six steps.
The enantiomeric purity of the aminocarnitine thus obtained (determined by means of conversion to the derivative obtained with o-phthalaldehyde and L-acetylcysteine and HPLC analysis, see J. Chromatography, 1987, 387, 255-265) was xe2x89xa799%.
The invention described herein also relates to the direct production of chiral aminocarnitine derivatives, that is to say with the advantage of allowing these compounds (of general formula corresponding to intermediate 7) to be obtained by first synthesising aminocarnitine and then derivatising it, as, in contrast, is envisaged in the above-cited patent applications MI98A001075 and PCT/IT99/00126.
In fact, with the insertion of step a) of the appropriate Y group, after hydrolysis (or catalytic hydrogenation, in the case of an ester removable with that technique) of intermediate 6, the desired formula 7 derivative is obtained.
Group X can be a leaving group selected, for example, from Br, I, Cl, OXxe2x80x2, where Xxe2x80x2 can be alkyl or aryl sulphonyl (in particular mesyl or tosyl);